1. Field of the Invention
The present invention relates to an image pickup device having a solid-state image pickup array.
2. Description of the Prior Art
A so-called portable video device having a compact video camera and a compact video tape recorder (VTR) has been recently developed vigorously. In the future, an advanced form of 8 mm video device which has a video camera and a VTR in union will be dominant.
The compactness of electronic equipment largely depends on semiconductor technology. A photo-electric converter of the video camera is being changed from an image pickup tube to a solid-state image pickup array as the semiconductor technology is advanced. The solid-state image pickup array has many advantages over the image pickup tube.
Because of the use of a solid-state device, the equipment is compact, of low power consumption, suitable for mass production and free from burning.
Because of the development of the technology of the solid-state image pickup array having such advantages and the development of a compact magnetic recording apparatus, a silver halide photographing technology which uses a prior art silver halide film as a record medium has been significantly affected by a magneto-photographing or electronic photographing technology which does not need a developing process.
An operation mode of the VTR in which an image of a moving subject is recorded on the VTR and the recorded image is displayed on a TV screen is called a movie video, and an operation mode in which an image of a still subject is recorded on the VTR and the recorded image is displayed on the TV screen or printed by a printer is called a still video. There is no great difference between signal formats for the movie video and the still video because both signals are converted to standard television signal formats.
However, the image of the subject is usually continuously picked up in the movie video mode while the image of the subject is momentarily picked up in the still video mode like a conventional camera device. Accordingly, operation responses to an iris, a shutter, an AGC and a white balance are substantially different in those two modes and driving methods of the solid-state image pickup array are also different. Therefore, the device cannot be used for both modes when the presently available optical system and signal processing system are used.
It is, therefore, desirable to provide a camera unit for the movie video mode and a camera unit for the still video mode separately, but a cost-performance requirements for such a device will be met only after the device has been fully diffused throughout the market. Thus, in the present stage, it is advantageous to use one camera unit for both modes.
A problem encountered when the camera unit is used for both the movie video mode and the still video mode is a method for accumulating and reading charges of the solid-state image pickup array. The solid-state image pickup array includes an X-Y address type MOS array, an image sensor, an interline type CCD (IL-CCD) array and a frame transfer type CCD (FT-CCD) array. The FT-CCD is specifically explained here.
The FT-CCD comprises an image pickup unit having a plurality of photo-electric conversion cells for converting an image of a subject to electric charges, a memory for temporarily storing signal charges from the image pickup unit, a horizontal shift register for reading out the signal charges from the memory in timed relation with a television synchronizing signal and an on-chip amplifier for amplifying the signal charges from the horizontal shift register to produce a signal voltage.
When such an FT-CCD is used as a movie camera, the image pickup unit photo-electrically converts the image for one field period and the photo-electrically converted signal charges are transferred to the memory in a vertical blanking period by vertical transfer pulses of several MHz. The signal charges in the memory are transferred to the horizontal shift register in the next field period one horizontal scan at a time during a horizontal blanking period, and read out from the on-chip amplifier as a CCD signal. During this period, the image pickup unit is in the photo-electric conversion state. Thus, the photo-electric conversion and the vertical transfer are repeated for each field so that a continuous video signal is produced.
When the FT-CCD described above is used as a still camera, the image flickers for the following reason. The TV signal consists of one frame of video signal and produces one frame of image from two fields (odd and even fields) by an interlace operation. Accordingly, one frame signal constructed by the photo-electric conversions at different times causes the image to flicker, particularly for a fast moving subject, and the quality of the image is degraded.
The following two methods have been proposed to resolve the above problem.
In the first method, only the odd field (or even field) signals are utilized. The odd field signal is used for the next even field signal. However, this method degrades vertical resolution and it is not appropriate for the still image.
In a second method, the number of vertical cells of the image pickup unit is doubled, and a second horizontal shift register is provided between the image pickup unit and the memory so that an even field signal and an odd field signal are simultaneously produced on a plane of the image pickup unit and they are sequentially read out so that the solid-state image pickup unit can be used for both the movie video mode and the still video mode. Such a solid-state image pickup device has been proposed in U.S. Pat. No. 4,486,783, issued Dec. 4, 1984, to Tanaka et al., and assigned in common herewith.
This image pickup device is shown in FIG. 1, and the operation thereof is briefly explained below.
In FIG. 1, numeral 1 denotes an image pickup unit having a plurality of photo-electric conversion cells arranged in a matrix, numeral 2 denotes a memory for storing signal charges produced by the image pickup unit 1, numeral 3 denotes a first horizontal shift register for reading out the signal charges produced by the image pickup unit 1, numeral 4 denotes a second horizontal shift register for reading out the signal charges stored in the memory 2, and numerals 5 and 6 denote on-chip amplifiers for amplifying the signals read from the first and second horizontal shift registers 3 and 4, respectively. The image pickup device forms, as a whole, a frame transfer type CCD array. The number of vertical cells of the image pickup unit 1 is 490 and the number of vertical cells of the memory 2 is 245.
In the movie video mode, the signal charges of two vertically adjacent picture cells of the image pickup unit 1 are sequentially added in the first shift register 3 and the combined signal charges are transferred to the memory 2 and read out from the second shift register 4 through the on-chip amplifier 6 as a signal 1C.
In the still video mode, the following two operation modes are possible.
In a first mode, like in the movie video mode, the signal charges of two vertically adjacent picture cells of the image pickup unit 1 are added and the combined signal charges are stored in the memory 2, and the signal 1C is read from the second shift register 4 through the on-chip amplifier 6. (This mode is aclled a field mode.)
In a second mode, the signal charges of the odd vertical cells of the image pickup unit 1 are read from the first horizontal shift register 3 and the signals charges of the even vertical cells are read from the second horizontal shift register. (This mode is called a frame mode.)
In the frame mode, vertical resolution in the still video mode is not degraded and a fully interlaced signal is produced.
To compare the field mode and the still mode, the signal charges in the frame mode in which the signal charges of the respective cells are directly read are approximately one half of those in the field mode in which the signal charges of the two adjacent cells are added. That is, sensitivity of the camera is one iris step lower.
The on-chip amplifiers 5 and 6 are usually MOS devices. Considering the fact that the MOS device has a poor low frequency noise characteristic and that the eyes of human beings are sensitive to low frequency noise, the reduction of the S/N ratio by one iris step is not negligible.
The number of horizontal cells of the solid-state image pickup array, which is 390 or 570 in the current state of art, is rather smaller than required, and in the future, the number of horizontal cells will be increased. In addition, it is expected that the present 2/3 inch optical system will be reduced to a 1/2 inch optical system and an 8 mm optical system in the future. Thus, the sensitivity of the camera is an important problem.
When the image pickup device of FIG. 1 is operated in the frame mode, the even field signal and the odd field signal are read through separate output amplifiers. Thus, if there is a difference between the characteristics of the output amplifiers, the output levels differ from each other. In such a case, the vertical resolution is significantly reduced.
It has been found that the following problem occurs when the outputs of the first and second shift registers are mixed. When the outputs of the output amplifiers 5 and 6 are mixed, the noises of the amplifiers 5 and 6 are superimposed, and the S/N ratio is reduced and the quality of image is degraded.